1. Field of the Invention
The present invention relates to a droplet ejecting apparatus that changes a volume of a pressure chamber and thereby ejects a droplet from a nozzle.
2. Discussion of Related Art
There is known a droplet ejecting device having a channel unit including a liquid supply manifold that supplies a liquid; a nozzle; and a pressure chamber provided between the liquid supply manifold and the nozzle. The droplet ejecting device changes a volume of the pressure chamber and thereby ejects a droplet of the liquid accommodated in the pressure chamber, from the nozzle. The droplet ejecting device is employed by, e.g., an ink jet printer head, as disclosed by Japanese Patent No. 3,128,857 (FIG. 1) or its corresponding U.S. Pat. No. 5,402,159.
Here, the construction and operation of the droplet ejecting device employed by the above-indicated printer head is described by reference to FIGS. 9 and 10 of the present application. FIGS. 9 and 10 show a droplet ejecting device 101 that is substantially identical with the droplet ejecting device disclosed by the above-indicated patent. More specifically described, FIG. 9 shows a condition of the droplet ejecting device 101 when a piezoelectric actuator unit 200 is not being operated; and FIG. 10 shows a condition of the droplet ejecting device 101 when the actuator unit 200 is being operated.
As shown in FIG. 9, the droplet ejecting device 101 includes a channel unit 100 having manifolds 111a, 112a communicating with an ink supply source, not shown, and functioning as an ink channel; a nozzle 154 through which droplets of the ink are ejected; and a pressure chamber 116 provided between the manifolds 111a, 112a and the nozzle 154. The actuator unit 200 that is deformable to change a volume of the pressure chamber 116, is adhered to an upper surface of the channel unit 100.
The channel unit 100 is provided by sheet members 143, 111, 112, 113, 114 that are stacked on each other. The sheet members 111, 112 have respective openings defining the respective manifolds 111a, 112a; and the sheet member 114 has an opening defining the pressure chamber 116. The actuator unit 200 is provided by individual electrodes 124 and common electrodes 125 that are alternately stacked on each other, and piezoelectric sheets 121a, 121b, 121c, 121d, 121e, 121f that are stacked alternately with the individual and common electrodes 124, 125.
The individual electrodes 124 are connected to an external positive electrode 130 via a through-hole 132; and the common electrodes 125 are connected to an external ground electrode 131 via a through-hole 133. Respective portions of the piezoelectric sheets 121b-121e that are sandwiched by the individual and common electrodes 124, 125 cooperate with each other to provide an active portion 161 that is opposed to the pressure chamber 116 of the channel unit 100 and is polarized, in advance, in a direction of thickness of the actuator unit 200.
When an electric field is produced by the individual and common electrodes 124, 125 in the same direction as the direction of polarization of the active portion 161, the active portion 161 is deformed, as shown in FIG. 10, in the direction of thickness of the actuator unit 200. As a result, the volume of the pressure chamber 116 is changed and a droplet 50 of ink is ejected from a nozzle 154.
In the droplet ejecting device 101, the individual electrodes 124 of the actuator unit 200 have substantially the same shape and size, in their plan view, as those of the pressure chamber 116, and the common electrodes 125 have such a size that the common electrodes 125 can each be opposed to a plurality of pressure chambers 116 of the channel unit 100. Therefore, respective “opposed” portions, Lx, of the individual and common electrodes 124, 125 that are opposed to each other in the direction of thickness of the actuator unit 200 have substantially the same shape in their plan view as that, Lc, of each pressure chamber 116. A total number of those opposed portions of the electrodes 124, 125 is equal to that of the pressure chambers 116. Meanwhile, a total area of those opposed portions of the electrodes 124, 125 is proportional to an electrostatic capacitance of the actuator unit 200; and the electrostatic capacitance is proportional to an amount of electric power consumption of a driver circuit that drives the actuator unit 200. If the electric power consumption of the driver circuit is high, the driver circuit needs to have a large size and accordingly it costs high. In addition, as the electric power consumption of the driver circuit increases, an amount of heat generated by it increases and accordingly it is needed to employ a heat sink having a large size. Thus, the overall size of the droplet ejecting device 101 is inevitably increased. Moreover, if the heat generated by the driver circuit is transmitted to the actuator unit 200, the operation of each active portion 161 is adversely influenced so that the each active portion 161 may not eject ink in a desired manner.